Laminates used for printed circuit boards for electronic and electric equipment include laminates wherein metallic foils are bonded with thermosetting adhesives such as thermosetting resin (hereinafter referred to as thermosetting laminates) and laminates wherein metallic foils are bonded with thermally fusible adhesives such as thermoplastic resin (hereinafter referred to as thermally fusible laminates).
Various methods for producing thermosetting laminates have been studied. Examples of these methods include a method for obtaining rigid laminates by thermosetting at a high temperature for several hours after pressing impregnated paper or impregnated glass cloths, and metallic foils using a multistage press or a vacuum press, a method for obtaining flexible laminates by thermosetting at a high temperature for some hours after laminating roll-type materials by sandwiching a pair of heated rolls, and a method for performing thermal lamination using a double-belt press instead of heated rolls. In these methods, a protective material may be sandwiched between pressing surfaces of the press and laminating materials to be heated and formed under pressure for the purpose of solving problems mentioned below. More particularly, protective materials may be used when the problems such as the appearance of scratches and scars on the surfaces of metallic foils (JP No. 60-109835) and the occurrence of warping of laminates when using a curing oven after thermal laminating (JP No. 4-89254), or the inhibition of smoother lamination caused by poorly evened with resin puddle impregnated paper or impregnated glass cloths, and the like arise.
When these thermosetting laminates are produced, the heating and forming under pressure temperature is virtually not higher than 200° C. Heat stress applied on laminating materials is small at such a low temperature, so that there is a low possibility of visual defects such as wrinkling which appear at the time of thermal lamination. On the contrary, thermal fusion is impossible unless heating at a temperature higher than the glass transition temperature (Tg) of the thermoplastic resin constituting bonding layers when producing thermally fusible laminates. On the other hand, thermoplastic resin in laminates for electronic and electric equipment constituting bonding layers is required to have a glass transition temperature at least not lower than 170° C. because of heating at a high temperature in the process of packaging parts. Accordingly, it is necessary to perform thermal lamination at a temperature not lower than 200° C. for thermal fusion. There was a problem that visual defects such as wrinkles may easily appear on the processed laminates at such a high temperature because of large change in thermal expansion and thermal shrinkage of the laminating materials.
Adhesives such as epoxy resin and acrylic resin adhesives are generally used for adhesive materials of the thermosetting laminates. Recently, however, heat resistance of substrates for on-vehicle systems used in bad environment such as high temperatures and heat resistance to solder reflow in lead-free soldering at a melting point higher than conventional solders by some ten degrees, and the like have been required as substrate characteristics, so that the conventional epoxy resin and acrylic resin adhesives have been regarded as insufficient in heat resistance.
Thus various kinds of polyimide adhesives combining heat resistance have been studied.
For example, a method for directly applying, drying, and curing polyimide varnishes and/or polyamic acid varnishes used as polyimide precursors onto metallic foils is known as a method for producing polyimide metallic foil laminates. In this method, solution-type varnishes were directly applied onto the metallic foils, so that the varnishes easily penetrated profiles on the metallic foil surfaces because of being in a solution state. This prevented small voids (micro voids) from occurring between the metallic foils and polyimide layers, even if the circuit pattern having a line width of 10 μm to 50 μm was formed. This method was, however, not satisfactory as circuit board materials because wrinkles, waviness, and warp, and the like appear on polyimide metal clad laminates caused by thermal shrinkage at the time of drying and curing the solvents when directly applying, drying, and curing the varnishes. Methods for producing polyimide metal laminates free from wrinkles, waviness, and warp have been proposed.
For example, JP No. 7-193349 discloses a method for producing a polyimide metal laminate by directly applying, drying, and curing a polyamic acid varnish used as a precursor for a thermoplastic polyimide varnish and/or a thermoplastic polyimide varnish onto a non-thermoplastic polyimide substrate to form a thermoplastic polyimide layer and bonding metallic foils to the surface of the thermoplastic polyimide layer by heating. Since the polyimide metal laminate obtained by the method is free from defectives such as wrinkling, waviness, and curls, the laminate is excellent as a circuit board material because of sufficiently strong peeling strength of the metallic foils and polyimide layer. Due to presence of micro voids with a diameter of 10 to 50 μm between the metallic foil and thermoplastic polyimide layer, the polyimide metal laminate was not good enough as a fine circuit pattern.
Recently, as electronic devices have been more and more downsized and reduced the weight, printed circuit boards, especially, flexible printed circuit boards constructed by the formation of a copper foil circuit on an insulating film have been demanded. Examples of common methods for producing flexible printed circuit boards include: forming a circuitry by continuously adhering a copper foil to be continuously provided onto an insulating film such as a polyimide film to be continuously provided with a thermosetting adhesive so that a circuitry may be formed on the copper foil by means such as etching; and forming a circuit on this copper foil by means such as etching by continuously laminating an adhesive film such as a polyimide film wherein a heat resistant thermoplastic adhesive has been previously formed on upper and lower surfaces to a copper foil by a continued application of heat.
Dimensional stability in these flexible printed circuit boards is strongly demanded to package high-density packaging parts used in cell phones and portable personal computers.
It is known that as described in JP No. 2-134241 and JP No. 10-126035, the dimensional stability highly depends on processing conditions when producing flexible laminates. For example, JP No. 10-126035 describes “Since an insulating film is lengthened and is laminated on a copper foil (a copper foil is hardly lengthened) in this state by the effects of tension in the adhesion process when the above-mentioned producing method is adopted, there are problems with the shrinking of a flexible printed circuit board because of the shrinkage of a polyimide film after the elongation distortion of the insulating film is released caused by partial elution of the copper foil when performing an etching processing to make a flexible printed circuit board by forming a circuitry. The coefficient of linear expansion of the polyimide film is greater than that of the copper foil when the polyimide film is laminated on the copper foil by heating, so that the polyimide film is laminated in the more elongated than the copper foil. Accordingly, when a circuitry is formed by the etching processing to make a flexible printed circuit board, there is a problem that the flexible printed circuit board is shrunk because of the shrinking of the polyimide film and releasing the elongation distortion due to partial elution of the copper foil at the time of etching process.”
This JP No. 2-134241 disclosed plasma treatment and laminating at a low temperature from 60° C. to 120° C. The plasma treatment, however, had problems which caused an increase in cost of the product and cannot be carried out in-line processing when laminating at a temperature higher than 120° C.
In the JP No. 10-126035, there were problems that laminating was performed within the low temperature range of 50° C. to 120° C., so that laminating at a temperature higher than 120° C. did not exert its effects.
For methods for obtaining laminates by thermal laminating, as described above, a method for obtaining laminates by pressing using a heated roll laminating machine, a multistage press or a vacuum press device and a method for thermally laminating roll-type materials using a double-belt press and the like are performed.
It is, however, impossible to continuously laminate as in being performed in Roll-to-Roll because the multistage press and vacuum press device are single acting. Although the double-belt press is capable of continuously laminating, initial costs and maintenance fee, and the like are expensive, compared with a single-acting press device. Endless belts for the double-belt press are obtained by welding and polishing both ends of a continuous steel belt, which are disposed, sandwiching press rolls, so that scars caused by a joint of each belt are transcribed and their transcription portions may not be usable as a product due to nonuniformity in adhesion. In addition, a problem arises that laminating materials are irregularly bonded when laminating is performed using belts with extreme irregularity in thickness in a cross-machine direction because of the transmission of pressure of the press rolls to the laminating materials through each of the belt. Thus, uniformity in the surfaces of the belts should be fully noted in the double-belt press system.
Since heated roll laminating machines are employed for thermosetting resin whose laminating temperature is relatively as low as 100° C., the machines respectively have a configuration of rubber-metal roll or rubber—rubber roll. This configuration enables to pressurize in the cross-machine direction without any irregularity in pressure because of elasticity of the rubber, which leads to uniform laminating. When thermally fusible laminating materials having heat resistance are used, the heated roll laminating machines with rubber-rolls cannot be used for such purposes because of deformation of the rubber due to heat being required to be 250° C. or higher at the time of laminating.
It is an object of the present invention to provide a method for producing a thermally fusible laminating material and a heat resistant laminate which comprises a metal material suitable as a flexible printed circuit board free from visual defects such as wrinkles and curls when thermal lamination is performed at a high temperature.
It is another object of the present invention to provide a method and a device for producing a heat resistant-flexible laminate which is capable of being uniformly heated and pressurized at the time of thermal-press forming, wherein the surface of the laminate is uniform and inter-layers are favorably bonded.